1. Technical Field of the Invention
The present invention relates to a device for preventing mixing of extrinsic noise provided in a signal transmission path for bi-directionally transmitting high-frequency signals as performed in a bi-directional CATV system with the aim of eliminating extrinsic noise flowing into up-stream signals, and the present invention further relates to a protector provided between such a signal transmission path and a signal input unit of a receiver apparatus such as a television receiver for protecting such connected apparatuses as the receiver by preventing breaking in of abnormal voltage.
2. Description of the Prior Art
One example of a circuitry view of a conventional protector is illustrated in FIG. 8. In FIG. 8, 1 denotes an input terminal to which a coaxial cable is connected, 2 an output terminal, wherein central electric conductors of both terminals are connected through a coupling capacitor 15 with an arrestor 5 and a choke coil 6 being provided on the input side so as to be formed between the conductor and respective grounds. An external electric conductor of the input terminal 1 is grounded while an external electric conductor of the output terminal 2 is grounded through a feed through capacitor 16. Noise-mixing ratio characteristics, which represent ratios of noise caught by the external electric conductor of a coaxial cable on an output side of this circuit being mixed in a transmission system on an input side, of this circuit will be characterized as illustrated in FIG. 10, wherein the noise-mixing ratio exceeds xe2x88x9240 dB at 10 MHz to 40 MHz.
It has been suggested for arrangements in which a coupling transformer is provided in a protector such as techniques as disclosed in Japanese Patent Examined Publication No. 5-54234 (1993) or Japanese Patent Examined Publication No. 5-50210 (1993). Of these two, the former Japanese Patent Examined Publication No. 5-54234 is arranged in that one ferrite element (coupling transformer) is obtained by winding two insulated wires between input and output terminals of a protector, wherein a primary side is connected to the input terminal side and a secondary side to the output terminal side. In the latter Japanese Patent Examined Publication No. 5-50210, an additional ferrite element that is obtained by winding two insulated wires is provided between input and output terminals wherein windings on primary sides are connected to core wire portions for input and output while winding on secondary sides are connected to external electric conductors for input and output.
In a bi-directional CATV system, it is generally the case that frequencies of not less than 70 MHz are used as down-stream frequency bands that are send out from central stations while frequencies of 10 MHz to 55 MHz are used as up-stream frequency bands for sending signals from respective homes of users in directions of the central stations. In the years, it has been aimed for achieving wide-band frequencies such that frequencies of up to 770 MHz, which are ground wave bands, are being transmitted, and it is further planned to transmit BS-IF bands exceeding such values. It is thus being demanded that apparatuses, which are interposed at some midpoints of signal transmission paths, exhibit favorable characteristics over an even broader range.
In view of this point, the protector of FIG. 8 exhibits poor noise-mixing ratio characteristics in such frequency bands and is not favorable since its effect for decreasing streamed noises is insufficient.
The above protector with the primary side of the ferrite element being connected to the input terminal side while the secondary side is connected to the output terminal side exhibits favorable signal passing characteristics in a range of 50 MHz to 300 MHz as represented by the signal passing characteristics and return loss characteristics in FIG. 9. However, when transmission is performed at frequencies of up to 770 MHz or in a range of 1035 to 1335 MHz that correspond to the BS-IF band, the protector cannot be used owing to the fact that traps are included in proximities of 1000 to 1200 MHz, and in case noise shall be caught by the external electric conductor on the output side, this noise will appear at the core wire on the input terminal side to cause generation of large streamed noises.
Also in case the primary side windings of ferrite elements are connected to input and output core wire portions and secondary windings to input and output external electric conductors, noise caught in the external electric conductors on the output side will not be completely eliminated such that it will appear on the core wire side on the input side.
The present invention has been made in view of the above problems, and it is a subject of the present invention to provide a device for preventing mixing of extrinsic noise and a protector that exhibit favorable signal passing characteristics for high-frequencies exceeding 770 MHz and in which noise caught on output sides are hardly transmitted to core wire units on input sides.
For solving the above subject, the device for preventing mixing of extrinsic noise according to the invention of claim 1 is a device for preventing mixing of extrinsic noise comprising an input terminal and an output terminal to which coaxial cables are connectable, with high-frequency signals being passed between the input terminal and the output terminal while direct current is prevented from passing on both, a core wire side and a ground side, wherein a pair of coupling transformers with primary windings and second windings is provided between the input terminal and the output terminal through cascade connection with one end of a primary winding on an up-stream side of the pair of coupling transformers being connected to a core wire of the input terminal while the other end thereof is connected to a ground on the input side, and wherein one end of a secondary winding on a down-stream side is connected to a core wire of the output terminal while the other end thereof is connected from the ground on the input side to a ground on the output side of which direct current is prevented from passing.
The invention of Claim 2 is characterized to be the device for preventing mixing of extrinsic noise as claimed in Claim 1, wherein the one end connected to the core wire of the input terminal of the up-stream coupling transformer and the one end connected to the core wire of the output terminal of the down-stream coupling transformer are connected through a choke coil.
The invention of Claim 3 is characterized to be the device for preventing mixing of extrinsic noise as claimed in Claim 1, wherein the pair of coupling transformers are connected such that turns ratios of the primary windings and the second windings are opposite to each other.
The invention of Claim 4 is characterized to be the device for preventing mixing of extrinsic noise as claimed in Claim 1, wherein frequency characteristics of the pair of coupling transformers are set such that one is of low-frequency passing type and the other of high-frequency passing type. The protector according to the invention of Claim 5 is a protector comprising the device for preventing mixing of extrinsic noise as claimed in Claim 1, the protector comprising an arrestor and a choke coil for preventing breaking in of abnormal voltage.